Beijing said that its navy had deployed two vessels: Lanzhou, a missile destroyer and Taizhou, a patrol boat, to warn the U.S. ship away from waters claimed by China. …

The United States’ dispatch of the USS Lassen had been planned weeks in advance, in what Washington has called an exercise of the right to freedom of navigation in international waters.

But on Tuesday China’s Foreign Ministry called the move illegal. The Lassen, meanwhile, has been heading back to its base in Yokosuka, Japan, according to an unidentified Pentagon official who spoke to The Times on the condition of anonymity.

The Navy destroyer had sailed within 12 nautical miles of Subi reef, where Beijing has built an artificial island. Satellite images have shown China has used large-scale dredging to reclaim land over live coral reefs, and construction on a runway has begun, although Chinese PresidentXi Jinpinghas said China is not pursuing militarization of the islands.

Andrew S. Erickson, an associate professor at the China Maritime Studies Institute at the U.S. Naval War College inRhode Island, said China is not entitled to a 12-nautical-mile territorial limit, according to the United Nations Convention on the Law of the Sea. The reefs are situated at a low-tide elevation, meaning beyond a 500-meter safety zone ships and aircraft are allowed to operate without consultation or permission.

Further thoughts from Andrew Erickson:

Beyond 500 meters of low-tide elevations (LTEs)—features naturally below water at high tide—foreign ships and aircraft are free to operate at will without consultation or permission, as USS Lassen has just done near Subi Reef. Unlike rocks or islands, under international law, LTEs such as Subi Reef are not entitled to the 12 nautical miles’ territorial waters or airspace that rocks or islands are. Rather, beyond a 500-meter safety zone, all high seas freedoms apply. These are the freedoms that USS Lassen has just exercised.

The USS Lassen’s positive action underscores U.S. commitment to maintaining an open global system with global commons that are free for all to use to the maximum extent permitted by international law, without favor, fear, or obstruction. Freedom of the Seas is a key element of this vital equation.

As can be seen from the peaceful, unimpeded nature of the USS Lassen’s operation, China and the U.S. share an interest in keeping the South China Sea’s vital sea lanes of stable and open.